internal/frontend: add sitemap URL to robots.txt

Change-Id: I06e5dc9e82956aaa3b40199b55b96a41b1bb271c
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/342485
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/internal/frontend/server.go b/internal/frontend/server.go
index 96c83e3..1cbb637 100644
--- a/internal/frontend/server.go
+++ b/internal/frontend/server.go
@@ -159,6 +159,7 @@
 		http.ServeContent(w, r, "", time.Time{}, strings.NewReader(`User-agent: *
 Disallow: /search?*
 Disallow: /fetch/*
+Sitemap: https://pkg.go.dev/sitemap/index.xml
 `))
 	}))
 }